The present disclosure relates to a crust breaking device for metal melts, in particular for aluminum melts, comprising a pneumatic cylinder having a cylinder housing, a piston which is axially displaceable within the cylinder housing, a piston rod, which is fastened to the piston and is guided through an opening at one end of the cylinder housing such that, by axial displacement of the piston, the piston rod is movable between a retracted final position and an extended final position. The device furthermore comprises an electronic control unit and a valve arrangement, which can be actuated by way of the electronic control unit and is connected via working lines to a front pressure chamber and a rear pressure chamber within the cylinder housing.
Crust breaking devices of the type mentioned above are commonly used in the aluminum industry for the production of aluminum. During this process, the aluminum is kept in a heatable container in order to produce a melt. In this process, a crust forms on the surface of the aluminum melt. So that further aluminum and/or additions can be added to the melt, it is necessary from time to time to break the crust. The crust is broken by means of a crust breaking device commonly installed above the container. The crust breaking device comprises a pneumatic cylinder having a piston rod, which is fastened to a piston that is arranged so as to be axially displaceable within the cylinder, in order to move the piston rod between a retracted final position and an extended final position, in which it breaks the crust. In order to make it easier to break the crust, the piston rod may be equipped with a breaking tool, for example a chisel or a pick. Once the crust has been broken, the piston rod returns to its retracted final position.
To actuate the crust breaking device, the rear pressure chamber or the front pressure chamber of the pneumatic cylinder is filled with compressed air by way of a valve arrangement connected to a compressed-air source, while the respective other pressure chamber is vented. The rising pressure in the respective pressure chamber filled with compressed air acts upon the piston, such that the piston is axially displaced. Accordingly, depending on the direction of the axial displacement of the piston, the piston rod is extended or retracted. In order to make it possible to switch over the direction of the axial displacement of the piston once the latter has reached a final position, the valve arrangement is furthermore connected to a control unit, which monitors the position of the piston and actuates the valve arrangement accordingly.
U.S. Pat. No. 6,436,270 discloses a method and a device for controlling the movement of a feeding and breaking chisel in an aluminum production plant, wherein breakage of the crust is detected by means of an AC circuit when the chisel comes into contact with the metal melt. During operation, a high current and a low DC current prevail between a container, which is in the form of a cathode and contains the aluminum melt, and an anode, such that an electrical contact is established between the chisel and the metal melt when the chisel breaks the crust. In this case, the crust acts as an electrical insulator, with electrical contact being prevented until the chisel has reached the melt. Once the electrical contact has been detected, the control unit switches over the supply of compressed air to the cylinder, as a result of which the chisel is withdrawn very quickly from the melt, such that a significant amount of heat is not introduced into the chisel. In this case, harmful heating of the cylinder is reduced to a minimum at the same time, this in turn resulting in lower maintenance and restoration costs. Although the disclosed method and the disclosed device make it possible to determine that the chisel has broken through the crust, they do not make it possible to determine the actual position of the chisel and accordingly of the piston, since the filling level of the melt and/or the thickness of the crust can vary.
Laid-open specification US 2009/0078110 discloses a pneumatic drive system comprising an axially displaceable output drive unit and control valve means, which are equipped with actuating means that are actuated depending on the position of the output drive unit. The actuating means are actuated when the output drive unit has reached a final position or a position just short thereof, with the control valve means being switched into another operating mode. The actuating means assigned to the control valve means are preferably in the form of mechanical response means, which are arranged directly on or in the drive housing. They comprise at least one displaceably mounted plunger member, which protrudes into the path of travel of the output drive unit, such that the output drive unit, upon axial displacement toward a final position, strikes against the response means, with the control valve means being switched. Said document furthermore proposes the use of response means whose actuation does not require physical contact with the output drive unit, for example reed switches or other position sensors. In this case, the control valve means are switched over by using an electric signal. However, the response means disclosed in said document comprise components, for example a displaceably mounted plunger part or electrically actuated sensors, which are susceptible to wear and/or failure, in particular in rough environments, for example the aluminum industry.
It is an object of the present disclosure to provide a crust breaking device of the type mentioned in the introduction which comprises position detection means that are less susceptible to wear and/or failure. The object is achieved by a crust breaking device having the features of the disclosure. Preferred embodiments of the disclosure are described in the claims.